Change withdrawal tests that were using TstFakeCredits to use real credits.

I kind of don't like that I cannot see exactly which methods are implemented on the interface. I'd have to know the Stringer interface to know that I then have to implement the String() string method.

On the other hand I see your point too - although I don't think it'll ever really change. In both cases if they changed the Stringer interface our code would no longer work (compile time errors)

It's fine